1. Statement of the Technical Field
The present invention is directed to the field of directional couplers, and more particularly, to directional couplers having a miniaturized design.
2. Description of the Related Art
Directional couplers are four-port circuits typically used for sampling of the input power for use in signal monitoring circuits. The sampled signal is typically measured to determine the power level, frequency, and/or signal shape (modulation) of the input signal. One typical directional coupler configuration is referred to as a hybrid coupler, a 3 dB coupler, a 3 dB hybrid coupler, a quadrature coupler, or a quadrature hybrid coupler, amongst other names. Regardless of how it is referred to, the quadrature hybrid coupler generally has the characteristics of dividing the input signal into two signals having equal powers and separated in phase by 90 degrees when the four ports are properly terminated.
Quadrature hybrid couplers are commonly implemented by using two edge coupled transmission lines. However, there are design challenges which arise when implementing quadrature hybrid couplers using planar circuit fabrication technologies, such as integrated circuit technologies, stripline technologies, and printed circuit board technologies. U.S. Pat. No. 7,741,929 to Hash discloses one type of miniature hybrid coupler which seeks to overcome some of these design challenges.
The design problems associated with implementing quadrature hybrid couplers using planar circuit fabrication technologies are compounded when wideband performance is a design goal. The reason for this generally relates to the need for additional coupler sections when implementing wideband coupler designs. The additional sections typically each have an electrical length of ¼ wavelength and therefore occupy a significant amount of space on a substrate. Moreover, the additional sections generally need to provide a relatively low amount of coupling. Low coupling is not conducive to compact layouts since it usually involves transmission lines traces having a relatively wide physical width and a relatively large space between coupled lines. Consequently, it has not been practical to implement wideband hybrid couplers in RF integrated circuits, except at millimeter wave. Instead, wideband hybrid couplers have been implemented using surface mount technology (SMT) components. These types of hybrid couplers can provide satisfactory performance, but are prohibitively large for many applications and cannot be practically implemented on RFICs.